This application relates to a type of “artificial lift system” commonly referred to as a “reciprocating rod lift”. A reciprocating rod lift that is used in extracting crude oil from an underground well generally comprises a surface pumping assembly, a downhole pump and a rod string, which is a series of sucker rods that connects the surface pumping assembly to the downhole pump. The rod string is connected to the surface pumping assembly via a polished rod at a “polished rod connection”. The polished rod is typically the uppermost rod in the rod string, and enables an efficient hydraulic seal to be made around the reciprocating rod string at the wellhead. The rod string extends into the ground within a tubing and casing which extends to the oil formation. The downhole pump is submerged within the oil formation. The surface pumping assembly provides a reciprocating upstroke and downstroke motion which allows the downhole pump to be filled and to lift a column of oil to the surface.
Some surface pumping assemblies comprise a hydraulic cylinder mounted on the wellhead. The hydraulic cylinder comprises a piston connected to the rod string via a polished rod connection. An associated hydraulic power unit (HPU) provides pressurized hydraulic fluid to lift, the piston within the cylinder, and thereby lift the attached rod string (on the upstroke). On the downstroke the piston and the attached rod string drops at a controlled rate (under gravity or by the application of pressure). The stroke length is the distance between the uppermost position and the lowest position of the polished rod during its reciprocating motion.
FIGS. 1-5 are simplified cross-sectional views illustrating some different types of (prior art) hydraulic cylinders that have been employed in reciprocating rod lifts. In each of the examples illustrated in FIGS. 1-5, a hydraulic power unit (not shown) can be used to supply pressurized fluid to the piston within the hydraulic cylinder to provide the upstroke and, optionally, the downstroke motion. A directional control valve is typically used on the HPU to reverse the direction of the piston when an upper or lower proximity sensor is triggered directly, or more commonly indirectly, by movement of the piston.
In pumping assembly 100 shown in FIG. 1, hydraulic cylinder 110 is mounted on mast 115 which is, in turn, mounted to wellhead 120 via wellhead mount 117. Piston 125 moves up and down in cylinder 110 under the influence of a hydraulic fluid introduced and discharged via port 112. Hydraulic fluid can also be introduced and discharged via port 114. Piston 125 is coupled to the upper end of cylinder rod 130, which is coupled to polished rod 140 via polished rod connection 145. Polished rod 140 is coupled to a sucker rod string not shown) which extends into the underground well. Proximity sensors 150 and 155 are mounted on mast 115, and are triggered by sensing metal protrusion 148 on polished rod connection 145 at the bottom and top of the stoke, respectively. Stuffing box 160 provides a seal around polished rod 140 to prevent, or at least reduce, leakage of the pumped fluid, such as crude oil. Designs similar to the pumping assembly of FIG. 1 are described, for example, in U.S. Pat. Nos. 7,762,321 and 8,235,107. In pumping assembly 100, the height of the structure is more than twice the maximum stroke length above the polished rod connection, and the cylinder rod is generally visible moving up and down within the mast. Typically support lines are needed to support or stabilize the mast.
FIG. 2 shows pumping assembly 200 in which hydraulic cylinder 210 is mounted directly on wellhead 220. Piston 225 moves up and down in cylinder 210 under the influence of a hydraulic fluid introduced and discharged via port 212. Hydraulic fluid can also be introduced and discharged via port 214. Piston 225 is coupled directly to the upper end of polished rod 240. Polished rod 240 is coupled to a rod string (not shown) which extends into the underground well. Cylinder seal gland assembly 209 provides a seal around polished rod 240. Proximity sensors (not shown in FIG. 2) can be incorporated into the pumping assembly and used to control the motion of the piston. This type of design is described, for example, in U.S. Pat. No. 4,503,752. With this design the height of the pumping assembly is only a bit greater than the stroke length, and the pumping assembly is relatively simple and compact. The polished rod contacts both the crude oil (or other fluid being pumped) and the hydraulic fluid within the cylinder, which can result in detrimental cross-contamination, even with the presence of seals. Also with this design, in order to access the well for servicing, as well as removing the surface pumping assembly, it is also generally necessary to uncouple the polished rod from the rod string. The connection point is typically within the well.
In pumping assembly 300 shown in FIG. 3 the mast is also eliminated. Hydraulic cylinder 310 is mounted to wellhead 320 via wellhead mount 317. In this case, hydraulic cylinder 310 is an annular cylinder with central, cylindrical cavity (or bore) 315 defined by interior cylindrical wall 313. Annular piston 325 moves up and down in cylinder 310 under the influence of a hydraulic fluid introduced and discharged via port 312. Hydraulic fluid can also be introduced and discharged via port 314. Piston 325 is coupled to hollow cylinder rod 330 that surrounds interior cylinder wall 313. Polished rod 340 is located axially in hydraulic cylinder 310 within cavity 315, and is coupled to an upper portion of hollow cylinder rod 330 via polished rod connection 345. Proximity sensors 350 and 355 are mounted on non-metal pipe 370 positioned adjacent hydraulic cylinder 310, and are triggered by sensing metal part 348 connected to polished rod connection 345 via string 372 positioned to move up and down within pipe 370. Stuffing box 360 provides a seal around polished rod 340 to prevent, or at least reduce, leakage of pumped fluid. In pumping assembly 300, when the piston is in the down position the height of the structure is only a little more than one stroke length above the wellhead, but at the top of the upstroke the hollow cylinder rod will extend approximately another stroke length above the wellhead, and will be visible moving up and down. The hollow cylinder rod will be exposed to the external environment as it protrudes above the cylinder. The annular cylinder and piston require additional seals as compared to the arrangements shown in FIGS. 1 and 2.
Other designs are known in which a combination of hydraulic cylinders (similar to those shown in FIG. 1) and accumulators are mounted on the wellhead via a wellhead mount and are coupled to move the polished rod up and down. For example, FIG. 4 shows pumping assembly 400 in which two hydraulic cylinders 410a and 410b are mounted to wellhead 420 via wellhead mount 417. Pistons 425a and 425b move up and down in cylinders 410a and 410b, respectively, under the influence of a hydraulic fluid introduced and discharged via ports 412a and 412b, respectively. Hydraulic fluid can also be introduced and discharged via ports 414a and 414b. Pistons 425a and 425b are coupled to cylinder rods 430a and 430b, respectively. Leveling plate 415 connects cylinder rods 430a and 430b. Polished rod 440 is located in a space between hydraulic cylinders 410a and 410b, and is coupled to an upper portion of leveling plate 415 via polished rod connection 445. Proximity sensors 450 and 455 are mounted on non-metal pipe 470 positioned adjacent hydraulic cylinders 410a and 410b, and are triggered by sensing metal part 448 connected to polished rod connection 445 via string 472 positioned to move up and down within pipe 470. Stuffing box 460 provides a seal around polished rod 440. U.S. Patent Application Publication No. 2010/0300679 describes a similar assembly with two hydraulic cylinders and two accumulators. As with the design shown in FIG. 3, when the pistons are in the down position the height of the structure is only a little more than one stroke length above the wellhead, but at the top of the upstroke the cylinder rods will extend approximately another stroke length above the wellhead, and will be visible moving up and down. The cylinder rods will be exposed to the external environment as they protrude above the cylinders.
The pumping assembly 500 shown in FIG. 5 includes a mechanism which aids in reducing the overall height of the assembly. Again hydraulic cylinder 510 is mounted to wellhead 520 via wellhead mount 517. Piston 525 moves up and down in cylinder 510 under the influence of a hydraulic fluid introduced and discharged via port 512. Hydraulic fluid can also be introduced and discharged via port 514. Piston 525 is coupled to the lower end of cylinder rod 530. Pumping assembly 500 comprises sheave/drum assembly 516 over which cable or belt 515 is looped. Cable 515 is secured to wellhead mount 517 and to polished rod connection 545. Polished rod 540 is coupled to a rod string (not shown) which extends into the underground well. The cable and sheave/drum assembly causes the polished rod to move twice the distance travelled by the piston, thus the height of the hydraulic cylinder is only approximately half of the stroke length. However, the cylinder (piston) has to lift approximately double the rod string load; thus requiring a higher operating pressure from the HPU for similar rod string load and cylinder geometry when compared to other conventional arrangements. The wellhead mount is therefore designed for twice the rod string load along with desired safety factors. Similar to pumping assembly 400, proximity sensors 550 and 555 are mounted on non-metal pipe 570 mounted adjacent hydraulic cylinder 510, and are triggered by sensing metal part 548 connected to string 572 positioned to move up and down within pipe 570 in conjunction with movement of polished rod 540. Stuffing box 560 provides a seal around polished rod 540. In this arrangement the cylinder is off-set from the rod string which can allow easier access to the well, as the pumping assembly may not need to be moved.
The present apparatus and method relate to a compact pumping assembly and associated system and method. The pumping assembly is relatively easy to install and maintain, and can offer other advantages in design and operation as described below.